1. Field of the Invention
This invention relates to an optical system that prints electronic images, such as may be used in an electronic photographic printer, and more particularly, to an optical system adapted to project light of a uniform illumination onto a photosensitive medium and adapted to transmit the graphical image with minimal registration problems.
2. Description of the Prior Art
There are numerous types of optical systems utilized in photographic printers. One such optical system is described in FIG. 1A, and is further described in commonly-assigned Patent Application, now U.S. Pat. No. 5,864,390 issued Jan. 26, 1999. In this optical system, a small source 10 projects emitted light 14 towards a parabolic mirror 12 that substantially collimates and reflects the colored light to a cylindrical lens 16. The cylindrical lens 16 causes the reflected light 15 to converge along a y-axis passing through a spatial light modulator 17 and forming a print line on a photosensitive medium 20.
One drawback of the one prior art optical system is an less than satisfactory image owing to the illumination drop-off of the image bearing light. The drop-off is due to the different path lengths the light must travel from small source 10 to photosensitive medium 20. It can be seen that emitted light 14 has a path length L.sub.1 along a central axis of the optical system and a second path length L.sub.2 at the farthest edge of parabolic mirror 12. Hence the distance to parabolic mirror 12 along the parabola varies relative to the small source 10. Therefore, since the closest point from the parabola is along a central axis A-A' and the farthest point from the parabola is along the edge of parabolic mirror 12, an illumination drop-off of emitted light 14 traveling along length L.sub.2 is greater than that of emitted light 14 that travels the length L.sub.1. This drop-off can be seen in FIG. 1B where a graph of light irradiance 22 with respect to the photosensitive medium 20 is shown by a curve 60.
While replacement of parabolic mirror 12 with a spherical mirror decreases the light drop-off problem as described in the aforementioned commonly assigned patent application, if the light source 10 is an area source, is to both the difference in path length and cosine fall-off, such as in the case of an LED light source, the optical system will exhibit a light drop-off due to cosine fall-off. Cosine fall-off exists because of the finite nature of an area light source as opposed to a point source. As the area light source has planar emitting dimensions, the angle of the emitted light 14 emitted from the area source varies along the mirror. The cosine fall-off in a system having a spherical mirror as opposed to a parabolic mirror is illustrated as graph 64 on FIG. 1B. Both graph 60 and graph 64 are less than a uniform illumination distribution of graph 66.
Further, if the light source 10 comprises standard light-emitting diodes (LEDs), the optical system will still have light irradiance problems owing to a structure of the LEDs as more fully shown on prior art FIG. 2A. An LED 42 is a semiconductor two-terminal solid-state device characterized primarily by its ability to pass current readily only in one direction.
The LED 42 is manufactured with a pad 46 deposited upon a top surface of the LED 42. The pad 46 is configured so as to create substantially uniform flow of electricity throughout the LED 42. The pad 46, however, blocks the light 54 emitted from a portion of the top surface of the LED 42.
When the LED 40 is used as the light source, as is shown in FIG. 1C, where like numerals designate like elements, the presence of the pad 46 alters the illumination distribution in the y-plane. The altered illumination distribution emitted by the LED 40 at the image plane B-B' is shown in FIG. 1D, which illustrates that the irradiance is decreased where the pad 46 obstructs the emitted light 54.
Turning now to FIG. 2A, a further drawback of using an LED light source is that a bonding wire 52 that connects the pad 46 to a substrate electrode 48 alters the transmission of the emitted light 54 from a portion of the top surface of the LED 42. Particularly, the emitted light 54 that is obstructed by the bonding wire 52 will have an altered angular light distribution as is shown in FIG. 2B by a curve 62 plotted as a graph of light emittanceon the abscissa versus function of angle .theta. across the surface of the LED 40, as the bonding wire 52 creates a strong shadow and a non-symmetrical, angular light-emission characteristic. Another drawback of using LEDs as a light source is that the bonding wire 52 is often coated with a passivation layer 56 that non-uniformity affects the emitted light 54. This is primarily owing to the tendency of the passivation layer 56 to clump at places along the bonding wire 52.
Further, LEDs that are edge emitters have a different angular light emission characteristic and contribute to a non-uniform illumination distribution in most optical systems in which they are incorporated.
Optical printers that utilize LEDs and lenses have another drawback in that they are often expensive. These printers usually project image-bearing light to an oscillating mirror, which directs the image-bearing light in a "fast-scan" direction. Motion of the photosensitive medium perpendicular to the oscillating mirror oscillation direction is used for the "slow-scan" direction. Each color of image-bearing light must be focused upon the photosensitive medium to form an acceptable image. Owing to the relative motion of the photosensitive medium in the "slow-scan" direction, however, expensive optical systems must be utilized to ensure exact registration of each color of an imaged line on the photosensitive medium.
Further, as exact registration is often assured by repositioning the optics or the photosensitive medium after each color of each line is imaged, there is sometimes a stopping and starting motion between the photosensitive medium and a print head containing the optics, thus the speed at which the optical printer can reproduce the image is decreased.
Accordingly it is an object of this invention to provide for an optical system that minimizes light drop-off from center to edge of the image.
It is a further object of this invention to provide an inexpensive optical system for a photographic printer.
It is still another object of invention to produce an optical system such that an image can be transferred to a photosensitive medium with minimal registration problems.
It is still a further object of invention to produce an optical system such that an image can be transferred to a photosensitive medium without mechanically stopping and starting.